As the Food and Drug Administration approaches a decision on a seven-year-old proposal to ban growth-promoting antibiotics from the diet of the nation's livestock, the agency official who knows the most about the subject finds himself in a curious position.

The issue pits livestock producers and drug manufacturers against health experts who fear that the practice endangers humans by developing bacteria that are immune to the most common and cheapest antibiotics. Both sides could hardly hope for a batter-versed regulator than Dr. Lester M. Crawford, who twice has served as head of the FDA's veterinary division and who has written scientific papers on the issue.

But Crawford's expertise has also led him to become an open supporter of the ban.

"I don't think he should have been put back in charge," said Jere E. Goyan, a former FDA commissioner, who was Crawford's boss in the late 1970s and who supports the ban. "His stand has been too public and too clear. On the other hand, . . . he is a person with expertise, and I would like to have someone with expertise making decisions.

"Anyone with expertise is bound to have prejudices one way or the other."

Goyan reflects a concern that often besets health and safety agencies: how to keep knowledgeable scientists involved in decisions without letting their biases taint a supposedly impartial process.

"The difficulty, when you get into some of these areas, is that you often will have a limited number of people who are experts," said John Topping, a staff director in the Environmental Protection Agency's air and radiation division. "When you've got hundreds of people who are experts, you can pretty readily have people take themselves out of the picture. When it's a more limited group, it's a constant problem."

"Nobody's a tabula rasa in approaching a situation," said Topping. "People are going to have opinions formed by experience."

Former FDA commissioner Donald Kennedy -- who originally proposed the ban -- disagreed with Goyan. "You have to ask yourself what constitutes impartial judgment," said Kennedy. "I don't happen to think that having once expressed a view on a position then closes off the ability of someone as able as Les Crawford to evaluate a related question in the light of further evidence. Sometimes you have to sacrifice a little bit of appearance to gain knowledge and familiarity."

While out of government in 1981, Crawford served as an unpaid chairman of an advisory committee to the Natural Resources Defense Council, which supports a ban. But about 15 years earlier, he had worked for American Cyanamid Co., one of the big manufacturers of antibiotic food additives.

Since he returned to the agency in 1982, Crawford has tried to stay away from the antibiotics issue. Recently, however, FDA Commissioner Frank Young decided to put the former veterinarian back in charge of developing evidence surrounding the question.

In so doing, some observers say, Young has opened the agency to accusations of bias on one of its touchiest issues. But at least one opponent of the ban saw no problem with Crawford's role. A spokesman for the Animal Health Institute, which represents major antibiotic manufacturers, said, "I think Lester Crawford is a fair-minded individual and a pretty good scientist and very well qualified to do his job."

When he decided to permit Crawford to work on the issue, Young noted that Crawford had worked "on behalf of both industry and consumer groups," and that his work was "scientific and scholarly in nature" and "short in duration with minimal financial interest."

"The subject of low-level penicillin-tetracycline in food-producing animal feed is a major public health issue . . . . The development of the issue will involve several components of the Center for Veterinary Medicine and therefore cannot proceed smoothly without strong scientific direction and management coordination from the center director . . . ," Young's statement said.

Among the evidence available to Crawford is a well-publicized study recently released by the Centers for Disease Control that traced an outbreak of food poisoning to antibiotic-resistant salmonella found in hamburger meat. This week, the FDA is also scheduled to release a report on the issue that Congress requested in the late 1970s.

Once Crawford compiles his evidence, Young will decide whether to proceed to a hearing before an agency administrative law judge. If he does, Crawford and his Center for Veterinary Medicine would present the case for a ban, while industry advocates argue against them. In that case, Young said, the ultimate decision-makers -- the commissioner and his aides -- would distance themselves from the issue until they receive the judge's report.

Crawford, who has been quoted as saying "I think it's time" for penicillin and tetracycline to be eliminated as a routine part of animal diets, recently said, "I suppose I could change my mind." But for now, he said, he sees his role as prosecutorial, trying to establish that antibiotics in animal feed can be identified as a direct cause of antibiotic resistance in bacteria harmful to humans.